Update to 4.20, boost headers complains C4668

Compilation pass in 4.19, with same warning message, but is of level “warning”.

Failed compilation in 4.20.3. with newly created blank c++ project, visual studio 2017.

"Error	C4668	'BOOST_ARCH_AMD64' is not defined as a preprocessor macro, replacing with '0' for '#if/#elif'	C420	L:\usr_win\boost_1_67_0\boost\predef\other\endian.h	151	"
"Error	C4668	'BOOST_RATIO_HAS_UNICODE_SUPPORT' is not defined as a preprocessor macro, replacing with '0' for '#if/#elif'	C420	L:\usr_win\boost_1_67_0\boost\ratio\detail\ratio_io.hpp	121	"

I checked previous project of UE4.19, it uses windows sdk 8.1, but new project of UE4.20, the windows sdk verison is 10.